Ion current measuring devices of the type mentioned above are known from German Patent Publications DE-OS 30 06 665 and from DE 19 50 24 02 A1.
The circuit arrangement of German Patent Publication DE-OS 30 06 665 uses a Zener diode or a varistor connected between the high voltage ignition source and the spark plug. A constant voltage drop is provided by the Zener diode or varistor. A capacitor is so connected to the circuit that the voltage drop across the Zener diode or varistor charges the capacitor, whereby the capacitor becomes a measuring voltage source. The capacitor may be connected in parallel to the Zener diode or the varistor may be connected to the capacitor through further diodes that are connected to permit the flow of a loading current.
The circuit arrangement of German Patent Publication DE-OS 30 06 665 is relatively simple, however it requires a large storage capacitor. Moreover, the measuring voltage is not constant, especially when measuring phases of long duration are involved which can occur when the engine runs at a low r.p.m. The voltage is not constant because the storage capacitor is discharged by the flow of the measuring current. Further, an undesirable current is superimposed on the current to be measured. The undesirable current is generated by the discharge of stray capacities in the spark plug, in the ignition coil or transformer and in the wiring of the circuit. A leakage current is also superimposed on the ion current to be measured. This leakage current is caused by the Zener diode which is used for limiting the voltage. Still another disadvantage of the just mentioned circuit arrangement is seen in that the resistor for measuring the current is connected in series with the storage capacitor, whereby a non-linearity is imposed on the measuring circuit. As a result, the voltage drop across the ion measuring path is no longer linearly proportional to the current value to be measured, whereby the known circuit is not very accurate.
The circuit arrangement by German Patent Publication 195 02 402 A1 discloses applying a positive voltage to the spark plug in order to sense an ion current having a negative polarity generated by the combustion. The positive voltage is generated by a capacitor which is connected to the low potential side of the secondary winding of the ignition transformer. This capacitor is charged through a diode by the electric ignition current in order to obtain a voltage with a positive polarity. A Zener diode is connected to make sure that the voltage across the capacitor is limited. The current flowing through the capacitor is the ion current to be measured. This ion current is supplied to a current voltage converter in order to convert the ion current into a voltage representing the ion current. Here again the disadvantage is seen in that the capacitor imposes a nonlinearity into the relationship between the measured voltage representing the ion current to be measured and the current itself because the negative terminal of the capacitor is maintained at a virtual ground potential.
Furthermore, both circuit arrangements described above have another disadvantage in that for the measuring of the ion current a voltage in the range of 70 to 400 V is necessary. Such relatively high voltage is applied to the ion measuring gap of the spark plug of the combustion engine. Such higher voltage makes the respective circuit component more expensive.
It is also known that the use of a measuring voltage of about 400 V greatly increases the sooting speed during cold starting of the combustion engine, whereby the spark gap is quickly contaminated as is, for example, described in European Patent Publication EP 0,305,347 B1.
German Patent Publication DE-OS 3,327,766 describes a circuit arrangement for measuring the ion current in which the measuring voltage is produced by an alternating voltage applied to the primary winding of the ignition coil or transformer, whereby the alternating voltage is stepped up in the ignition transformer to a higher voltage level, and whereby frequencies in the range of 10 kHz to 100 kHz are used. The ion current signal causes an amplitude modulation of the alternating currents as generated in the secondary winding of the ignition coil. Such a system has the disadvantage that on the one hand it requires the use of filters to filter out the ion current signal having a useful frequency within the range of 100 Hz to 20 kHz in order to separate this ion current frequency from the carrier signal. On the other hand, the ion current characteristic curve is non-symmetric or non-linear, whereby the alternating generation of the ion current is subject to non-linear distortions. This non-symmetry or the respective non-linear distortion is the result of the higher movability of the negative load carriers relative to the positive ions. As a result, where non-symmetric electrodes are used for the spark gap, as is generally the case in conventional spark plugs, a larger current occurs when the slower positive charge carriers, namely the ions, travel toward the larger electrode.
U.S. Pat. No. 5,483,818 (Brandt et al.) discloses a circuit arrangement for detection of an ion current in which the low potential side or end of the secondary circuit of the secondary ignition winding is connected through a resistor to the inverting input of an operational amplifier, while the non-inverting input of this amplifier is connected to a reference voltage of about 40 V. This operational amplifier includes a resistor so connected that the amplifier functions as an inverting amplifier so that the reference voltage for measuring the ion current is connected to the secondary circuit and thus to the spark plug as a measuring voltage. The measuring voltage which represents the ion current or which is an ion current proportional signal at the output of the operational amplifier, is supplied to a threshold circuit for evaluation.
For discharging the ignition current generated during ignition two Zener diodes are connected in series and to the secondary circuit of the ignition transformer. A closed loop control circuit is provided for compensating for the leakage current that occurs in the Zener diodes. This leakage current falsifies the current component that represents the ion current. This closed loop compensation circuit is also controlled from the output of an operational amplifier. The closed loop compensation circuit comprises a further operational amplifier with respective resistors and a capacitor in the closed loop control circuit. The disadvantage of the circuit arrangement of U.S. Pat. No. 5,483,818 is seen in that its construction is involved so that its manufacturing costs are rather high.